Electrodeposition bath

ABSTRACT

An electrodeposition bath for depositing a Sn alloy onto a Cu substrate containing a Zn salt. This bath is particularly useful for Pb free alloy, such as a Sn--Bi alloy. The presence of Zn in the elctrodeposition bath greatly influence the bath behaviour and the characteristics of the deposited alloy, even if no Zn is codeposited on the Cu substrate.

TECHNICAL FIELD

The present invention relates to an electrodeposition bath; moreparticularly to a bath for electrodepositing a tin (Sn) based alloy.

BACKGROUND OF THE INVENTION

The use of tin based solder alloys is common in electronic applications,particularly in the manufacturing of printed circuit boards (PCB), forassembly of components onto the boards, providing mechanical andelectrical connection. These tin solder alloys are useful in joiningintegrated circuit chips to chip carriers and substrates, joining chipcarriers to substrates, and joining circuitization lands and pads inmultilayer printed circuit boards.

In the manufacturing of a microelectronic package, it is common practiceto attach a component onto a printed circuit board or the like, forexample by surface mounting utilizing a solder connection. For thispurpose, the board features a circuit trace including a pad thatconstitutes a first surface for the connection; similarly, the componentincludes a second surface, for example a contact.

The interconnection method comprises the steps of applying a solderalloy on the Cu substrate, typically onto the pad included in theprinted circuit board.

The electronic components to be joined with the board are then broughtinto contact with the solder layer. The solder alloy is heated to causethe solder alloy to melt and reflow; heating may be by vapor phasereflow, infrared reflow, laser reflow, or the like. Upon cooling, thesolder alloy resolidifies and bonds to the surfaces to complete theconnection. The solder connection not only physically attaches thecomponent to the board, but also electrically connects the trace on theboard and the contact of the component to conduct electrical current toand from the component for processing.

Tin-lead (Sn--Pb) alloys have been used for most electronic solderingoperations. These alloys have been selected because of their mechanicalstrength, low relative cost, electrical conductivity and excellentwetting characteristics; wettability is an indication of how completelyand quickly the molten solder can cover a solid surface. In addition,Sn--Pb alloys provide a low melting temperature, which is important inelectronic applications because many components and printed circuitboards use materials that are easily damaged by exposure to hightemperature during manufacture or assembly.

However, lead has been recognized as a health hazard, being toxic forworkers and for the environment; recently governments have begun to urgethe electronic industry to find alternatives to lead in order to reduceelectronic industry worker lead exposure and reduce the amount of leadwaste going back into the environment.

Lead presence in the soldering alloys is particularly critical in thecase of application for manufacturing the most recent generation ofC-MOS; in fact the details are so thin in this kind of board, that theemission of α particles from the emitting radioisotope present in thelead can provoke serious problems for the device.

Tin-Bismuth (Sn--Bi) solder alloys were investigated as alternatives toSn--Pb solder alloys. Electrodeposition of such Sn--Bi alloys fromdifferent electrolytes and in particular from alkyl sulphonate baths isknown in the art, as described in Surf. & Coat. Tech--Vol. 27, 151-166(1986)--Y. N. Sadana, R. N. Gedye, S. Ali. Electrodeposition of Sn--Bialloys onto a PCB with an alkyl sulphonate electrolyte is also describedin U.S. Pat. No. 5,039,576.

A different lead-free solder alloy for microelectronic applications isdescribed in EP-A 94108684.5. Such document discloses solder alloyscontaining more than 90% weight percent tin (Sn), and an effectiveamount of silver (Ag) and bismuth (Bi), optionally with Antimony (Sb) orwith Sb and copper (Cu). Different methods for obtaining the describedalloys, including electrodeposition, are mentioned.

Lead-free solder alloys known in the art present however some drawbacks.They exhibit poor soldering and metallurgical properties, that is smallpeel strength and low creep resistance. Particularly, they have shownpoor mechanical properties at temperatures of the type typicallyencountered by microelectronic packages during use. A Sn--Bi alloy, forexample, when electrodeposited onto copper of PCBs from alkylsulphonateor other electrolytes shows some difficulties related to the lowwettability and stability.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a technique whichalleviates the above drawbacks.

According to the present invention we provide an electrodeposition bathfor electrodepositing a tin-bismuth (Sn--Bi) alloy onto a copper (Cu)substrate the bath comprising:

zinc (Zn) salt.

Further, according to the present invention we provide a method forelectrodepositing a tin-bismuth (Sn--Bi) solder alloy onto a coppersubstrate using an electrodeposition bath containing zinc (Zn) salt.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 shows a graphic comparison of the potentiodynamic behaviourbetween a prior art electrodeposition bath and a bath according to apreferred embodiment of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The method of the present invention is particularly useful in themanufacturing of electronic modules, when electronic components need tobe soldered onto a Cu substrate. According to a preferred embodiment theSn alloy is lead (Pb) free and is a tin-bismuth (Sn--Bi) alloy.

According to the preferred embodiment, for example in the manufacturingof a printed circuit board, the solder alloy is applied to a Cusubstrate, typically onto the pad included in the printed circuit board;alternatively, the solder alloy could be deposited onto electricalcontacts of the component to be joined with the Cu substrate, such as anintegrated circuit chip. The board is immersed in the bath, spaced apartfrom a suitable counterelectrode (anode). An electrical current, isapplied to the anode for cathodically reducing the salts in the bath(for example Si, Bi and Cu) to their respective metals and therebydeposit the desired solder alloy onto the board. The conductivesubstrate is kept immersed for a time sufficient to deposit e solderalloy coating of the desired thickness and composition upon thesubstrate. The substrate is subsequently withdrawn from theelectroplating bath. The plated conductive substrate is thereafterwashed thoroughly as quickly as possible, to minimize staining.

In the preferred embodiment of the present invention the bath solutionincludes an alkyl-sulfonate electrolyte, typically methane sulphonate,in an amount sufficient to inhibit precipitation of alloy components,that is to maintain stability of the bath.

The soluble components are present in the bath in amounts sufficient todeposit a solder alloy onto a conductive substrate.

The bath includes an anode, for example an inert anode formed ofplatinum-plated titanium, and a substrate cathode immersed in theelectrolyte solution and an appropriate source of electrical energy forelectrodepositing the solder alloy onto a conductive substrate. As apossible alternative, a soluble anode, well known in the art, could beused.

In the preferred embodiment there has been used a bath solution which iscommercially available. It is produced by LeaRonal Inc., Freeport, N.Y.,USA. The composition and the characteristics of the LeaRonal bathsolution are as follows:

    ______________________________________                                        Sn (as stannous methane sulphonate)                                                                  8                                                      g/l                                                                           Bi (as bismuth methane sulphonate)                                                                   20      g/l                                            Methane sulphonic acid 500     ml/l                                           Finish treatment additive                                                                            60      ml/l                                           pH                     0.3                                                    Temperature            20-40°                                                                         C.                                             Current density        0.5-4   A/dm.sup.2                                     ______________________________________                                    

To the above solution is added Zn metasulphonate salt, with Zn contentin the range 5-30 g/l, maintaining all the other parameters unchanged.With the above described baths it was possible to obtain byelectrodeposition a thick deposit, of thickness greater than 50 micron,in reasonable times for the industrial application. The obtained depositis of uniform and near eutectic composition, it showed a goodwettability and had a good adherence to copper, which is important forthe reflowing process.

The addition to the bath of zinc methanesulphonate has been proved tostrongly influence the bath behaviour, although zinc was not dischargedonto the cathodic surface. For example FIG. 1 reports thepotentiodynamic behaviour, obtained with a potentiostat where thevoltage was imposed between the working electrode and a referenceelectrode (and corrected to the Standard Hydrogen Electrode SHE), thescanning rate of the voltage was 0.5 mV/s, the temperature 25 C. Theruns were made after deposition of the Sn--Bi alloy for 5 min. Threesolutions were examined: the LeaRonal Sn--Bi without addition of Zn salt(curve 1); the same solution with addition of Zn metalsulphonate with Zncontent of 5 g/l (curve 2); the same with Zn content of 30/g/l (curve3). From a comparison of the curves it is possible to observe a decreaseof the rest potential when Zn is added to the bath, a better behaviourat low cathodic current densities and an increase of the voltage atcurrent densities at which electrodeposition is normally made.

Zinc is known to be harmful if present in the tin alloy, beingpreferentially oxidized and giving large quantities of dross when thesolder is stirred. However according to the invention the zinc, while itis present in the bath solution, is not codeposited on the substratetogether with Sn--Bi.

One of the main advantages provided by the presence of zinc salts in theelectrodeposition bath is that the wettability of the solder alloy isgreatly enhanced.

After the solder deposition process, the electrical leads of thecircuitized substrate are then brought into contact with the contacts ofthe component, wherein at least one surface is coated with theelectrodeposited solder alloy. While the substrate current leads and thecomponent are maintained in contact, the solder alloy is heated to causethe solder alloy to melt and reflow; heating may be by vapor phasereflow, infrared reflow, laser reflow, or the like. The assembly isusually heated at a temperature greater than 140° C., and preferablygreater than 160° C. Upon cooling, the solder alloy resolidifies andbonds to the surfaces to complete the connection. The solder connectionnot only physically attaches the component to the board, but alsoelectrically connects the trace on the board and the contact of thecomponent to conduct electrical current to and from the component forprocessing.

During laboratory tests a comparison has been made between the samplesobtained from the original LeaRonal Sn--Bi solution and from the samewith added zinc methane sulphonate with zinc content of 15 g/l, after 5min of deposition time at 20 mA/cm², onto copper substrates, soldered at250 C, in an industrial oven with N₂ atmosphere, by XPS-ESCAspectroscopy.

In the case of the Sn--Bi alloy electrodeposited from the bath withoutzinc, we observed a great quantity of oxygen at the surface (54%), thatwas maintained also in depth (hydrates or basic salts), as observed bydepth profiling with sputtering.

At a depth of 0.045 micron a typical composition was:

Sn 47%, Bi 11%, O 42%;

at 0.09 micron the composition was:

Sn 52%, Bi 19%, O 30%;

at 0.135 micron:

Sn 63%, Bi 23%, O 14%.

In the case of the Sn--Bi alloy electrodeposited from the bath with zincmethanesulphonate (Zn 15 g/l), we observed a great quantity of oxygenonly at the surface (46%), which decreased in depth, as observed bydepth profiling with sputtering.

At a depth of 0.045 micron a typical composition was:

Sn 31%, Bi 65%, O 4%;

at 0.09 micron:

Sn 49%, Bi 48%, O 3%;

at 0.135 micron:

Sn 64%, Bi 33%, O 3%.

These figures show that the addition of zinc methanesulphonate to theelectrodeposition bath caused a substantial decrease of hydrates orbasic salt coprecipitation during SnBi alloy electrodeposition,increasing the deposit wettability and decreasing the oxidation at thesurface of the Sn--Bi alloy after reflowing.

Also the mechanical properties of the soldered Sn--Bi alloyelectrodeposited with a bath according to the present invention havebeen proved by laboratory tests to be significantly enhanced. Forexample the peel strength of the soldered joint was increased.

We claim:
 1. An electrodeposition bath for electrodepositing a tin-bismuth alloy onto a copper substrate, said bath comprising:zinc salt; tin salt; and bismuth salt; wherein said zinc salt is present in an amount effective to increase wettability of the electrodeposited alloy and decrease oxidation at the surface of the electrodeposited alloy after solder reflow.
 2. The electrodeposition bath of claim 1 wherein said zinc salt is zinc metasulphonate.
 3. The electrodeposition bath of claim 2 wherein said zinc content is in the range of from about 5 g/l to about 30 g/l.
 4. The electrodeposition bath of claim 3 wherein said zinc content is about 15 g/l.
 5. The electrodeposition bath of claim 1 wherein said bath is lead-free.
 6. A method for electrodepositing a tin-bismuth alloy onto a copper substrate comprising the steps of:providing an electrodeposition bath containing zinc salt, tin salt, and bismuth salt; providing a copper substrate; and electrodepositing a tin-bismuth alloy onto said copper substrate; wherein said zinc salt is present in an amount effective to increase wettability of the electrodeposited alloy and decrease oxidation at the surface of the electrodeposited alloy after solder reflow.
 7. The method of claim 6 wherein the current density applied during said electrodepositing is within the range of from about 0.5 A/dm² to about 4 A/dm².
 8. The method of claim 7 wherein said zinc salt is zinc metalsulphonate.
 9. The method of claim 8 wherein the zinc content is within the range of from about 5 g/l to about 30 g/l.
 10. The method of claim 9 wherein said zinc content is about 15 g/l.
 11. The method of claim 6 wherein said electrodeposition bath is lead-free during said electrodepositing of said tin-bismuth alloy onto said copper substrate.
 12. A soldering method for soldering electronic components onto a copper substrate using a tin-bismuth solder alloy comprising the steps of:electrodepositing said tin-bismuth solder alloy on said copper substrate using an electrodeposition bath containing zinc salt, tin salt, and bismuth salt; bringing said electronic components in contact with said tin-bismuth solder alloy; and reflowing said tin-bismuth solder alloy to join said electronic components to said copper substrate; wherein said zinc salt is present in an amount effective to increase wettability of the electrodeposited alloy and decrease oxidation at the surface of the electrodeposited alloy after solder reflow.
 13. The soldering method of claim 12 wherein said electrodeposition bath is lead-free during said electrodepositing of said tin-bismuth solder alloy on said copper substrate. 